in desperate need of help...

i had previously written a post on this issue, but in need of further assistance. My 90 bonneville has a brand new battery that keeps dying, and a new alternator. I am thinking that there is a short somewhere, but can anyone help me with a sure fire way to fix this or at least a sure answer that will lead me to the problem? I am trying to sell the car, and if it doesn't start for the test drives, then it probably isn't going to have very good chances of selling...i deeply appreciate any help...